RAYMOND: NEW AND OLD SILURIAN TRILOBITES. 15 



cave border on either shield. Eyes very large. Type, Bumastus 

 harriensis Murchison. Ordovician and Silurian. North America. 

 Silurian. Europe. 



Actinolobus Eichwald. Axial lobe narrow, cephalon short, pygi- 

 diura long, with very wide concave border. Eyes large. Type, 

 Illaenus atavus Eichwald. Ordovician, Russia. Silurian. Russia 

 and United States. 



lUaenoides Weller Axial lobe with a width between that of Actino- 

 lobus and Bumastus, eyes very small and far forward, narrow con- 

 cave border on pygidium. Type, lUaenoides trilobus Weller. Silurian. 

 United States. 



The above classification is designed to separate the species with 

 long more or less flattened shields from the more typical illaenids with 

 short and abruptly deflected cephalon and pygidium. The first two 

 genera are modifications of the central Illaenus type, the other three 

 of the more flattened Dysplanus group. In defining the subfamily 

 Bumastinae as I have, all the forms with a more or less Isotelus-like 

 pygidium are removed from the typical Illaenus group. Among the 

 small species constituting the earliest of the Bumastinae one finds 

 species like B. glohosus Billings, B. bellevillensis Raymond and Narra- 

 way, and a few others, which lack a concave border. On the other 

 hand, so large a Middle Ordovician Bumastus, as B. indeterviinatvs 

 (Walcott), the type of which is figured (Plate 2) for the first time, has a 

 distinctly concave border. It is very possibly true that the small 

 species mentioned above should be given a distinct name and placed 

 in the lUaeninae, but it still seems somewhat early to take so radical 

 a step. 



Description of species. 



We owe to Professor Weller a complete and careful description of 

 the Illaenidae of the Chicago area, and, as he had access to collections 

 made at Racine and near Milwaukee, his description in large measure 

 covers the Wisconsin area also. In the large collections which I have 

 been able to examine, I have, however, found a few specimens more 

 perfect than those previously described, and also a few new species. 

 When first studying the excellent figures given by Weller, one is struck 

 by the apparent triviality of the specific characteristics employed in 

 the discrimination of the species, but with a large collection, it is 

 found that the characteristics are remarkably constant. The study 

 of these illaenids is unusually interesting, in fact, for it seems to be 



